WASHINGTON (AP) -- Even in an era when former U.S. officials routinely profit from business ventures linked to their public service, recently retired National Security Agency chief Keith Alexander raised eyebrows when he disclosed he is working on patents for what he calls a game-changing cybersecurity model.

Alexander had access to the nation's deepest secrets about cyberwarfare, and he spent the last several years warning about cyberthreats to private industry. Critics questioned whether he was cashing in on classified information he learned at NSA, and they asked why he didn't deploy his new ideas while he led the government's cyber defenses.

In an interview Tuesday with The Associated Press, Alexander sought to answer those questions and rebut what he says are misimpressions about the new company he leads, IronNet Cybersecurity.

The retired four-star general, who departed in March after nine years as NSA director, said there is nothing improper about working in his field of expertise. He also led the military's cyber command.

"If I retired from the Army as a brain surgeon, wouldn't it be OK for me to go into private practice and make money doing brain surgery?" he asked. "I'm a cyber guy. Can't I go to work and do cyber stuff?"

His firm is developing as many as 10 patents, he said, and has secured contracts with three clients he declines to name. The technological innovations in the new patents came from an unidentified partner, he said, and are not specifically derived from anything he learned at NSA or cyber command. Alexander also filed three patents while he was NSA director, but the taxpayers own the rights to those, and if he wants to use them he must apply for a license, as anyone could, he said.

Lawyers at NSA and his private lawyers-- including former FBI Director Robert Mueller, now with the Wilmer Hale law firm in Washington -- have told him he is on firm legal footing, Alexander said.

"I've been in government for 40 years; I fully understand the importance and sanctity of classified material," said Alexander, who ended his career dealing with the stunning revelations of former NSA systems analyst Edward Snowden.

Reports of his firm charging $1 million a month for consulting services are not accurate, he said, though he declined to disclose his firm's fees.

"That number was inflated from the beginning," he said.

Alexander acknowledged he stands to become a rich man, if his firm perfects what he calls a new behavioral model designed to detect sophisticated hackers who penetrate a corporate network using techniques whose pattern is not previously known.

"If it actually works, this will be worth a lot," he said.

He said he didn't develop the idea while working at NSA, but even if he had, that agency lacks the authority to defend private networks. The agency mainly protects secret networks with classified tools and information sharing that does not exist in the private sector.

Alexander's path from a government job to a related private venture is well worn. His predecessor, Michael Hayden, is a consultant with Chertoff Group, the security consulting and private equity firm led by Michael Chertoff, the former Homeland Security chief, who is also working with Alexander on the cyber venture.

The three NSA directors prior to Alexander and Hayden have each worked in the defense industry after leaving office, including retired Vice Admiral Michael McConnell, who has earned more than $1 million a year in recent years as vice chairman of intelligence contractor Booz Allen, according to the company's proxy statements.

Participants in Alexander's new venture include James Heath, a former NSA adviser, and Russell Richardson, a former Army intelligence official. John "Chris," Inglis, who retired in March as the NSA deputy director, is helping part time.

"Actually knowing something about cybersecurity is a rare commodity," said James Lewis, a cyber expert at the Center for Strategic and International Studies in Washington. "He's got the skills and he's trying to figure out how to monetize them. It's hardly unusual."